Rocky Mountain Spotted Fever
Also called: RMSF
Pathogen
Rickettsia rickettsii
Carried by
American dog tick, Rocky Mountain wood tick, brown dog tick
Rocky Mountain Spotted Fever is one of the most serious tick-borne diseases in the US. Despite the name, most cases occur in south-central and southeastern states, with severe outbreaks along the Arizona–Mexico border driven by the brown dog tick. It can become life-threatening within days, so treatment is started on clinical suspicion — before test results return.
RMSF is a genuine emergency. If you have a sudden fever and spreading rash after possible tick exposure, seek care right away and mention the tick bite.
Symptoms
- Sudden fever and severe headache
- Rash that often begins on wrists and ankles, then spreads to palms and soles
- Muscle pain, nausea, vomiting
- Rapid progression to serious illness if untreated
Testing
RMSF is a clinical diagnosis — treatment should never be delayed for testing. Paired antibody tests confirm it after the fact; PCR or biopsy of a rash/eschar can help.
Labs often used in the Lyme community:
See our Testing Guide to understand your options.
Geography
Highest case counts historically in North Carolina, Oklahoma, Arkansas, Missouri, and Tennessee. The severe, high-fatality brown-dog-tick outbreaks cluster in Arizona and along the US–Mexico border, disproportionately affecting children.
Explore the Where Ticks Live map to see what's carried in your region.
Treatment overview
Doxycycline, immediately, for all ages — delay increases the risk of death. This is the clearest 'don't wait' scenario in tick-borne illness.
Frequently asked questions
Why treat before test results come back?
RMSF can be fatal within days, and antibody tests often aren't positive until after the dangerous window. The standard of care is to start doxycycline on clinical suspicion.